The modern leatherback sea turtle Dermochelys coriacea possesses a unique, flexible mosaic carapace adapted for deep diving. The evolutionary origins of this structure remain poorly understood because of the fragmentary nature of the fossil record. Here we describe Ueloca colemanorum n. gen. n. sp., a novel ridgeless leatherback turtle from the lower Oligocene Glendon Limestone of Alabama, USA. This specimen (MSC 49490) represents one of the most complete fossil leatherback carapaces known and provides new insight into dermochelyid diversity and shell evolution. The carapace is composed of parallel rows of enlarged ossicles interspersed with smaller ossicles arranged in distinctive floriform patterns. Histological analysis reveals a cancellous core with an external cortex but no basal cortex, a condition also observed in Dermochelys coriacea. Phylogenetic analysis places Ueloca within a clade of ridgeless leatherbacks (Uelocini), which diverged from ridge-bearing Dermochelyini in the Eocene. Our revised phylogeny suggests that Uelocini and Dermochelyini represent the two main lineages of dermochelyines, both characterized by mosaic carapaces that undergo parallel morphological trends. These findings clarify the deep history of leatherback shell evolution and emphasize the need for additional well-preserved fossils to further refine dermochelyid systematics.